Method for generating pollution credits while processing reactive metals

ABSTRACT

This invention relates to a method for generating pollution credits while processing molten magnesium, aluminum, lithium, and alloys of such metals by contacting the molten metal or alloy with a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/780,256, filed Feb. 9, 2001, and now allowed,and claims priority to U.S. Provisional Patent Application No.60/202,169, filed May 4, 2000, each of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates in one aspect to a method for generatingpollution credits while processing molten reactive metals such asmagnesium, aluminum, lithium, and alloys of such metals.

BACKGROUND OF THE INVENTION

[0003] Molded parts made of magnesium (or its alloys) are findingincreasing use as components in the automotive and aerospace industries.These parts are typically manufactured in a foundry, where the magnesiumis heated to a molten state to a temperature as high as 1400° F. (800°C.), and the resulting molten magnesium is poured into molds or dies toform ingots or castings. During this casting process, protection of themagnesium from atmospheric air is essential to prevent a spontaneousexothermic reaction from occurring between the reactive metal and theoxygen in the air. Protection from air is also necessary to minimize thepropensity of reactive magnesium vapors to sublime from the molten metalbath to cooler portions of a casting apparatus. In either situation, anextremely hot magnesium fire can result within a few seconds of airexposure, potentially causing extensive property damage and seriousinjury or loss of human life. Similarly, aluminum, lithium, and alloysof such metals are highly reactive in molten form necessitatingprotection from atmospheric air.

[0004] Various methods have been investigated to minimize the exposureof molten magnesium to air. See J. W. Fruehling et al., Transactions ofthe American Foundry Society, Proceeding of the 73^(rd) Annual Meeting,May 5-9, 1969, 77 (1969). The two most viable methods for effectivelyseparating molten magnesium from air are the use of salt fluxes and theuse of cover gases (sometimes referred to as “protective atmospheres”).A salt flux is fluid at the magnesium melt temperature and iteffectively forms a thin impervious film on the surface of themagnesium, thus preventing the magnesium from reacting with oxygen inthe air. However, the use of salt fluxes presents several disadvantages.First, the flux film itself can oxidize in the atmosphere to harden intoa thick deposit of complex metal oxide/chlorides, which is easilycracked to expose molten magnesium to the atmosphere. Second, the saltfluxes are typically hygroscopic and, as such, can form salt inclusionsin the metal surface which can lead to corrosion. Third, fumes and dustparticles from fluxes can cause serious corrosion problems to ferrousmetals in the foundry. Fourth, salt sludge can form in the bottom of thecrucible. Fifth, and not least, removal of such fluxes from the surfaceof cast magnesium parts can be difficult.

[0005] As a result, there has been a shift from using salt fluxes tousing cover gases to inert molten magnesium. Cover gases can bedescribed as one of two types: inert cover gases and reactive covergases. Inert cover gases can be non-reactive (e.g., argon or helium) orslowly reactive (e.g., nitrogen, which reacts slowly with moltenmagnesium to form Mg₃N₂). For inert cover gases to be effective, airmust be essentially excluded to minimize the possibility of metalignition, i.e., the system must be essentially closed. To utilize such aclosed system, workers either have to be equipped with a cumbersomeself-contained breathing apparatus or they have to be located outside ofthe dimensions of the processing area (e.g., by using remote control).Another limitation of inert cover gases is that they are incapable ofpreventing molten metal from subliming.

[0006] Reactive cover gases are gases used at low concentration in acarrier gas, normally ambient air, that react with the molten magnesiumat its surface to produce a nearly invisible, thermodynamically stablefilm. By forming such a tight film, the aerial oxygen is effectivelyseparated from the surface of the molten magnesium, thus preventingmetal ignition and minimizing metal sublimation.

[0007] The use of various reactive cover gases to protect moltenmagnesium from ignition has been investigated as early as the late1920s. An atmosphere containing CO₂ is innocuous and economical yetforms a protective film on a magnesium surface which can preventignition for over 1 hour at 650 ° C. However, the CO₂-based films formedare dull in appearance and unstable, especially in the presence of highlevels of air, and consequently offer little protection for themagnesium surface from ambient oxygen. In effect, the CO₂ behaves morelike an inert cover gas than a reactive cover gas.

[0008] U.S. Pat. No. 4,770,697 (Zurecki) discloses the use ofdichlorodifluoromethane as a blanketing atmosphere or cover gas formolten aluminum-lithium alloys. U.S. Pat. Nos. 6,398,844 and 6,521,018(both Hobbs et al.) disclose blanketing gases used with non-ferrousmetals and alloys with reduced Global Warming Potentials, but which arevery toxic to workers and/or corrosive to process equipment.

[0009] SO₂ has been investigated in the past as a reactive cover gas, asSO₂ reacts with molten magnesium to form a thin, nearly invisible filmof magnesium oxysulfides. SO₂ is low in cost and is effective at levelsof less than 1% in air in protecting molten magnesium from ignition.However, SO₂ is very toxic and consequently requires significantmeasures to protect workers from exposure (permissible exposure levelsare only 2 ppm by volume or 5 mg/m³ by volume). Another problem with SO₂is its reactivity with water in humid air to produce very corrosiveacids (H₂SO₄ and H₂SO₃). These acids can attack unprotected workers andcasting equipment, and they also contribute significantly to acid rainpollution when vented out of the foundry. SO₂ also has a tendency toform reactive deposits with magnesium which produce metal eruptions fromthe furnace (especially when SO₂ concentrations in the air are allowedto drift too high). Though SO₂ has been used commercially on a largescale for the casting of magnesium alloys, these drawbacks have led somemanufacturers to ban its use.

[0010] Fluorine-containing reactive cover gases provide an inertatmosphere which is normally very stable to chemical and thermalbreakdown. However, such normally stable gases will decompose uponcontact with a molten magnesium surface to form a thin,thermodynamically stable magnesium oxide/fluoride protective film. U.S.Pat. No. 1,972,317 (Reimers et. al.) describes the use offluorine-containing compounds which boil, sublime or decompose attemperatures below about 750° C. to produce a fluorine-containingatmosphere which inhibits the oxidation of molten magnesium. Suitablecompounds listed include gases, liquids or solids such as BF₃, NF₃,SiF₄, PF₅, SF₆, SO₂F₂, (CClF₂)₂, HF, NH₄F and NH₄PF₆. The use of BF₃,SF₆, CF₄ and (CClF₂)₂ as fluorine-containing reactive cover gases isdisclosed in J. W. Fruehling et al., described supra.

[0011] Each of these fluorine-containing compounds has one or moredeficiencies. Though used commercially and effectively at lower levelsthan SO₂, BF₃ is toxic and corrosive and can be potentially explosivewith molten magnesium. NF₃, SiF₄, PF₅, SO₂F₂ and HF are also toxic andcorrosive. NH₄F and NH₄PF₆ are solids which sublime upon heating to formtoxic and corrosive vapors. CF₄ has a very long atmospheric lifetime.(CClF₂)₂, a chlorofluorocarbon, has a very high ozone depletionpotential (ODP). The ODP of a compound is usually defined as the totalsteady-state ozone destruction, vertically integrated over thestratosphere, resulting from the unit mass emission of that compoundrelative to that for a unit mass emission of CFC-11 (CCl₃F). SeeSeinfeld, J. H. and S. N. Pandis, Atmospheric Chemistry and Physics:From Air Pollution to Climate Change, John Wiley & Sons, Inc., New York,(1998). Currently, there are efforts underway to phase out theproduction of substances that have high ODPs, includingchlorofluorocarbons and HCFCs, in accordance with the Montreal Protocol.UNEP (United Nations Environment Programme), Montreal Protocol onSubstances that Deplete the Ozone Layer and its attendant amendments,Nairobi, Kenya, (1987).

[0012] Until recently, SF₆ was considered the optimum reactive cover gasfor magnesium. SF₆ is effective yet safe (essentially inert, odorless,low in toxicity, nonflammable and not corrosive to equipment). It can beused effectively at low concentrations either in air (<1%) or in CO₂ toform a very thin film of magnesium oxyfluorides and oxysulfides on thesurface of molten magnesium. This magnesiumoxide/fluoride/sulfide/sulfur oxide film is far superior at protectingthe magnesium from a vigorous exothermic oxidation reaction than is themagnesium oxide film inherently present on the metal surface. Themagnesium oxide/fluoride/sulfide/sulfur oxide film is sufficiently thin(i.e., nearly invisible to the naked eye) that the metal surface appearsto be metallic. This superior protection is believed to result from thegreater thermodynamic stability of a nonporous magnesium sulfide/sulfuroxide and/or magnesium oxide/fluoride film as compared to the stabilityof a thick porous film of either magnesium oxide, sulfide or fluoridealone.

[0013] In a typical molten magnesium process employing a reactive covergas, only a small portion of the gas passed over the molten magnesium isactually consumed to form that film, with the remaining gas beingexhausted to the atmosphere. Efforts to capture and recycle the excessSF₆ are difficult and expensive due to its very low concentrations inthe high volumes of exhaust stream. Efficient thermal oxidizingequipment would be required to remove the SF₆ from the exhaust stream,adding significantly to production costs. Product costs can also beconsiderable, as SF₆ is the most expensive commercially used reactivecover gas.

[0014] However, perhaps the greatest concern with SF₆ is its verysignificant global warming potential (3200 year atmospheric lifetime,and about 22,200 times the global warming potential of carbon dioxide).At the December 1997 Kyoto Summit in Japan, representatives from 160countries drafted a legally binding agreement containing limits forgreenhouse gas emissions. The agreement covers six gases, including SF₆,and includes a commitment to lower the total emissions of these gases bythe year 2010 to levels 5.2% below their total emissions in 1990. UNEP(United Nations Environment Programme), Kyoto Protocol to the UnitedNations Framework Convention on Climate Change, Nairobi, Kenya, 1997.

[0015] As no new replacement for SF₆ is yet commercially available,efforts are underway to reinvigorate SO₂, as SO₂ has essentially noglobal warming potential (despite its other considerable drawbacks). SeeH. Gjestland, P. Bakke, H. Westengen, and D. Magers, Gas protection ofmolten magnesium alloys: SO₂ as a replacement for SF₆. Presented atconference on Metallurgie du Magnesium et Recherche d'Allegement dansI”Industrie des Transports, International Magnesium Association (IMA)and Pole de Recherche et de Devleoppment Industriel du Magnesium(PREDIMAG) Clermond-Ferrand, France, October 1996.

[0016] The data in TABLE 1 summarize selected safety and environmentallimitations of compounds currently known to be useful in the protectionof molten magnesium. Numbers followed by an asterisk (*) areparticularly problematic with regard to safety and/or environmentaleffects. TABLE 1 Global Ozone Atmo- Warming Depletion spheric Potential-Potential- Com- Exposure Lifetime⁽³⁾ GWP⁽³⁾ ODP⁽³⁾ pound Guideline⁽¹⁾(yrs) (100 yr ITH) (CFC-11 = 1) SO₂ 2 ppmv* BF₃ 1 ppmv* NF₃ 10 ppmv* 740 10800* SiF₄ 2.5 mg/m³ as F* PF₅ 2.5 mg/m³ as F* SF₆ 1000 ppmv 320022200* SO₂F₂ 5 ppmv* (CClF₂)₂ 1000 ppmv 300 9800* 0.85* HF 3 ppmvceiling* NH₄F 2.5 mg/m³ as F* NH₄PF₆ corrosive, causes burns⁽²⁾* CF₄Moderately toxic 50000* 5700* by inhalation CHClF₂ 1000 ppmv 11.8 1900*0.055*

[0017] As each of these compounds presents either a significant safetyor an environmental concern, the search continues to identify newreactive cover gases for protecting molten magnesium, aluminum, lithium,and alloys of such metals which are simultaneously effective, safe,environmentally acceptable, and cost-effective.

SUMMARY OF THE INVENTION

[0018] This invention relates in one aspect to a method for generatingpollution credits while processing molten reactive metals and alloys ofsuch metals, e.g., magnesium, aluminum, lithium, and alloys of one ormore of such metals. Reactive metals are metals (and alloys) which aresensitive to destructive, vigorous oxidation in air. In brief summary,the invention provides a method for generating pollution creditscomprising:

[0019] (a) treating molten reactive metal or alloy of such metal toprotect said metal or alloy from reacting with oxygen in air by (1)providing molten metal or alloy and (2) exposing said metal or alloy toa gaseous mixture comprising a fluorocarbon selected from the groupconsisting of perfluoroketones, hydrofluoroketones, and mixtures thereofto yield protected metal or alloy having a protective film thereon; and

[0020] (b) taking allocation of pollution credits.

[0021] In one embodiment, this invention employs a method for treatingmolten reactive metal or alloy to protect it from reacting with oxygenin air. The method comprises providing molten reactive metal or alloyand exposing it to a gaseous mixture comprising a fluorocarbon selectedfrom the group consisting of perfluoroketones, hydrofluoroketones, andmixtures thereof. The gaseous mixture may further comprise a carriergas. The carrier gas may be selected from the group consisting of air,carbon dioxide, argon, nitrogen and mixtures thereof.

[0022] One advantage of the present invention over the known art is thatthe Global Warming Potentials of perfluoroketones and hydrofluoroketonesare quite low. Therefore, the present inventive process is moreenvironmentally friendly. By employing the method for treating orprotecting molten reactive metals or alloys which is described herein,processors who handle molten reactive metals or alloys will be able toproduce unit quantities of such metals and alloys and parts containingsuch metals and alloys as before while generating much smallerquantities of materials exhibiting significant GWP contribution or otherenvironmentally desirable effect.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

[0023] Fluorocarbons used in the present invention includeperfluoroketones (PFKs), and hydrofluoroketones (HFKs) which incorporatelimited amounts of hydrogen in their structures. These fluorocarbons canbe effective as reactive cover gases to protect reactive molten reactivemetals such as molten magnesium from ignition. As is the case with knownfluorine-containing reactive cover gases, these fluorocarbons can reactwith the molten metal surface to produce a protective surface film, thuspreventing ignition of the molten metal. For convenience, the followingdescription refers to molten magnesium, but it should be understood thatthe invention is also applicable to other reactive molten metals andalloys, including aluminum, lithium and alloys of one or more ofmagnesium, aluminum or lithium.

[0024] For the protection of molten magnesium from ignition,fluorocarbons of the present invention are desirable alternatives to themost commonly used cover gas currently, SF₆. The fluorocarbons of thepresent invention are low GWP fluorocarbon alternatives to SF₆, i.e.,the fluorocarbons of the present invention have measurably lower globalwarming potential relative to SF₆ (i.e., significantly less than 22,200)and are not significantly worse in atmospheric lifetime, ozone depletionpotential, or toxicity properties.

[0025] Perfluorinated ketones (PFKs) useful in the present inventioninclude ketones which are fully fluorinated, i.e., all of the hydrogenatoms in the carbon backbone have been replaced with fluorine atoms. Thecarbon backbone can be linear, branched, or cyclic, or combinationsthereof, and will preferably have about 5 to about 9 carbon atoms.Representative examples of perfluorinated ketone compounds suitable foruse in the processes and compositions of the invention includeCF₃CF₂C(O)CF(CF₃)₂, (CF₃)₂CFC(O)CF(CF₃)₂, CF₃(CF₂)₂C(O)CF(CF₃)₂,CF₃(CF₂)₃C(O)CF(CF₃)₂, CF₃(CF₂)₅C(O)CF₃, CF₃CF₂C(O)CF₂CF₂CF₃,CF₃C(O)CF(CF₃)₂, perflurocyclohexanone, and mixtures thereof. Inaddition to demonstrating reactive cover gas performance, perfluorinatedketones can offer additional important benefits in safety of use and inenvironmental properties. For example, CF₃CF₂C(O)CF(CF₃)₂ has low acutetoxicity, based on short-term inhalation tests with mice exposed forfour hours at a concentration of 100,000 ppm in air. Also based onphotolysis studies at 300 nm CF₃CF₂C(O)CF(CF₃)₂ has an estimatedatmospheric lifetime of 5 days. Other perfluorinated ketones showsimilar absorbances and thus are expected to have similar atmosphericlifetimes. As a result of their rapid degradation in the loweratmosphere, the perfluorinated ketones have short atmospheric lifetimesand would not be expected to contribute significantly to global warming(i.e., low global warming potentials). Perfluorinated ketones which arestraight chain or cyclic can be prepared as described in U.S. Pat. No.5,466,877 (Moore et al.) which in turn can be derived from thefluorinated esters described in U.S. Pat. No. 5,399,718 (Costello etal.). Perfluorinated ketones that are branched can be prepared asdescribed in U.S. Pat. No. 3,185,734 (Fawcett et al.). All of thesepatents are incorporated by reference in their entirety.

[0026] Hydrofluoroketones (HFKs) that are useful in the presentinvention include those ketones having only fluorine and hydrogen atomsattached to the carbon backbone. The carbon backbone can be linear,branched, or cyclic, or combinations thereof, and preferably will haveabout 4 to about 7 carbon atoms. Representative examples ofhydrofluoroketone compounds suitable for use in the processes andcompositions of this invention include: HCF₂CF₂C(O)CF(CF₃)₂,CF₃C(O)CH₂C(O)CF₃, C₂H₅C(O)CF(CF₃)₂, CF₂CF₂C(O)CH₃, (CF₃)₂CFC(O)CH₃,CF₃CF₂C(O)CHF₂, CF₃CF₂C(O)CH₂F, CF₃CF₂C(O)CH₂CF₃, CF₃CF₂C(O)CH₂CH₃,CF₃CF₂C(O)CH₂CHF₂, CF₃CF₂C(O)CH₂CHF₂, CF₃CF₂C(O)CH₂CH₂F,CF₃CF₂C(O)CHFCH₃, CF₃CF₂C(O)CHFCHF₂, CF₃CF₂C(O)CHFCH₂F,CF₃CF₂C(O)CF₂CH₃, CF₃CF₂C(O)CF₂CHF₂, CF₃CF₂C(O)CF₂CH₂F,(CF₃)₂CFC(O)CHF₂, (CF₃)₂CFC(O)CH₂F, CF₃CF(CH₂F)C(O)CHF₂,CF₃CF(CH₂F)C(O)CH₂F, and CF₃CF(CH₂F)C(O)CF₃. Some hydrofluoroketones canbe prepared by reacting a fluorinated acid with a Grignard reagent suchas an alkylmagnesium bromide in an aprotic solvent, as described inJapanese Patent No. 2,869,432. For example CF₂CF₂C(O)CH₃ can be preparedby reacting pentafluoropropionic acid with magnesium methyl bromide indibutyl ether. Other hydrofluoroketones can be prepared by reacting apartially fluorinated acyl fluoride with hexafluoropropylene in ananhydrous environment in the presence of fluoride ion at elevatedtemperature, as described in U.S. patent application Ser. No. 09/619306(herein incorporated by reference). For example, HCF₂CF₂C(O)CF(CF₃)₂ canbe prepared by oxidizing tetrafluoropropanol with acidic dichromate,then reacting the resulting HC₂H₄COOH with benzotrichloride to formHC₂H₄C(O)Cl, converting the acyl chloride to the acyl fluoride byreaction with anhydrous sodium fluoride, and then reacting theHC₂H₄C(O)F with hexafluoropropylene under pressure.

[0027] The gaseous mixture that comprises a fluorocarbon selected fromthe group consisting of perfluoroketones and hydrofluoroketones furthercomprises a carrier gas or carrier gases. Some possible carrier gasesinclude air, CO₂, argon, nitrogen and mixtures thereof. Preferably, thecarrier gas that is used with the perfluroketones is dry air.

[0028] The gaseous mixture comprises a minor amount of the fluorocarbonand a major amount of the carrier gas. Preferably, the gaseous mixtureconsists of less than about 1% of the fluorocarbon and the balancecarrier gas. More preferably, the gaseous mixture contains less than0.5% by volume (most preferably less that 0.1% by volume) fluorocarbon,selected from the group consisting of perfluoroketones,hydrofluoroketones and mixtures thereof.

[0029] In order to keep the protective layer on the magnesium, thegaseous mixture is continuously, or nearly continuously, fed to thesurface of the magnesium. Small breaks in the thin protective layer canthen be healed without the possibility of such small breaks exposingmolten magnesium to the air and initiating a fire.

[0030] A cover gas composition is of low toxicity both as it is appliedto the molten magnesium and as it is emitted from the process in whichit is used. Cover gases comprising low toxicity hydrofluoroketones andperfluoroketones, and mixtures thereof, will be safe mixtures as appliedto magnesium. However, all fluorine containing cover gas compositionproduce measurable amounts of hydrogen fluoride upon contact with themolten magnesium due to some level of thermal degradation and reactionwith magnesium at temperatures of 650 to 800° C. Hydrogen fluoride iscorrosive and toxic and its concentration in the emitted gas should beminimized. A preferred cover gas composition will, therefore, produceminimal hydrogen fluoride. See Examples, below.

[0031] Atmospheric lifetimes and global warming potentials for severalfluorocarbons used in accordance with this invention, along withcompounds currently known to be useful in the protection of moltenmagnesium as comparative examples, are presented in TABLE 2. TABLE 2Atmospheric Global Warming GWP Lifetime Potential (GWP) relativeCompound (years)⁽¹⁾ (100 year ITH)⁽¹⁾ to SF₆ HydrofluoroketoneHCF₂CF₂C(O)CF(CF₃)₂ ≦0.1⁽⁴⁾ ≦10⁽⁴⁾ 0.0005 PerfluoroketoneC₂F₅C(O)CF(CF₃)₂ 0.02⁽²⁾ 1⁽²⁾ 0.00005 Comparative Compounds:Hydrofluorocarbons FCH₂CF₃ 13.6 1600 0.07 CF_(3CHFCHFCF) ₂CF₃ 17.1 17000.08 CF₃CHFCF₃ 36.5 3800 0.17 HCF₂CF₃ 32.6 3800 0.17 SegregatedHydrofluoroethers C₄F₉OCH₃ 5.0 390 0.02 C₄F₉OC₂H₅ 0.8 55 0.002C₃F₇CF(OC₂H₅)CF(CF₃)₂ 2.5⁽²⁾ 210⁽²⁾ 0.01 Non-SegregatedHydrofluoroethers HCF₂OCF₂CF₂OCF₂H 7⁽³⁾ 1725⁽³⁾ 0.08 HCF₂OCF₂OC₂F₄OCF₂H7.1⁽³⁾ 1840⁽³⁾ 0.08 Other Fluorochemicals SF₆ 3200 22200 1.00 NF₃ 74010800 0.49 CClF₂CClF₂ 300 9800 0.44 CF₄ 50000 5700 0.26 C₂F₆ 10,00011,400 0.51

[0032] The perfluoroketones and hydrofluoroketones used in accordancewith the invention have much lower global warming potential (GWP) thanthe fluorocarbons known in the art such as SF₆, hydrofluorocarbons, andhydrofluoroethers. As used herein, “GWP” is a relative measure of thewarming potential of a compound based on the structure of the compound.The GWP of a compound, as defined by the Intergovernmental Panel onClimate Change (IPCC) in 1990 and updated in Scientific Assessment ofOzone Depletion: 1998 (World Meteorological Organization, ScientificAssessment of Ozone Depletion: 1998, Global Ozone Research andMonitoring Project—Report No. 44, Geneva, 1999), is calculated as thewarming due to the release of 1 kilogram of a compound relative to thewarming due to the release of 1 kilogram of CO₂ over a specifiedintegration time horizon (ITH).${{GWP}_{x}\left( t^{\prime} \right)} = \frac{\int_{0}^{ITH}{F_{x}C_{ox}^{{{- t}/\tau}\quad x}{t}}}{\int_{0}^{ITH}{F_{{CO}_{2}}{C_{{CO}_{2}}(t)}{t}}}$

[0033] where F is the radiative forcing per unit mass of a compound (thechange in the flux of radiation through the atmosphere due to the IRabsorbance of that compound), C is the atmospheric concentration of acompound, τ is the atmospheric lifetime of a compound, t is time and xis the compound of interest.

[0034] The commonly accepted ITH is 100 years representing a compromisebetween short-term effects (20 years) and longer-term effects (500 yearsor longer). The concentration of an organic compound, x, in theatmosphere is assumed to follow pseudo first order kinetics (i.e.,exponential decay). The concentration of CO₂ over that same timeinterval incorporates a more complex model for the exchange and removalof CO₂ from the atmosphere (the Bern carbon cycle model).

[0035] Carbonyl compounds such as aldehydes and ketones have been shownto have measurable photolysis rates in the lower atmosphere resulting invery short atmospheric lifetimes. Compounds such as formaldehyde,acetaldehyde, propionaldehyde, isobutyraldehyde, n-butyraldehyde,acetone, 2-butanone, 2-pentanone and 3-pentanone have atmosphericlifetimes by photolysis ranging from 4 hours to 38 days (Martinez, R.D., et al., 1992, Atmospheric Environment, 26, 785-792, and Seinfeld, J.H. and Pandis, S. N., Atmospheric Chemistry and Physics, John Wiley &Sons, New York, p. 288, 1998). CF₃CF₂C(O)CF(CF₃)₂ has an atmosphericlifetime of approximately 5 days based on photolysis studies at 300 nm.Other perfluoroketones and hydrofluoroketones show similar absorbancesnear 300 nm and are expected to have similar atmospheric lifetimes.

[0036] The very short lifetimes of the perfluoroketones andhydrofluoroketones lead to very low GWPs. A measured IR cross-sectionwas used to calculate the radiative forcing value for CF₃CF₂C(O)CF(CF₃)₂using the method of Pinnock, et al. (J. Geophys. Res., 100, 23227,1995). Using this radiative forcing value and the 5-day atmosphericlifetime the GWP (100 year ITH) for CF₃CF₂C(O)CF(CF₃)₂ is 1. Assuming amaximum atmospheric lifetime of 38 days and infrared absorbance similarto that of CF₃CF₂C(O)CF(CF₃)₂ the GWP for HCF₂CF₂C(O)CF(CF₃)₂ iscalculated to be 9. The perfluoroketones and hydrofluoroketones of theinvention typically have a GWP less than about 10.

[0037] As a result of their rapid degradation in the lower atmosphere,the perfluoroketones and hydrofluoroketones have short lifetimes andwould not be expected to contribute significantly to global warming. Thelow GWP of the perfluoroketones make them well suited for use as anenvironmentally preferred cover gas.

[0038] Also, the PFKs and HFKs of this invention can react more fullywith molten magnesium than does SF₆. As a result less unreacted covergas can be emitted to the atmosphere; less cover gas can be required toproduce a comparably performing protective film; or both. Consequently,useful concentrations of the cover gas can be lowered, thus reducing theglobal warming impact. The full substitution of fluorocarbons of thepresent invention for SF₆ can be accomplished without increasing therisk to worker safety since these materials (PFKs, and HFKs) are of lowtoxicity, are non-flammable, and are generally very innocuous materials.

[0039] Substitution for SF₆ with a PFK, or HFK, alone or as a mixturethereof, can provide protection of molten magnesium in variousprocesses, such as magnesium refining, alloying, formation of ingots orcasting of parts. This substitution can be straightforward and canprovide the same utility as a reactive cover gas that only SF₆ doescurrently. Surface films produced with the fluorocarbons of the presentinvention can be more stable to higher temperatures than those formedwith SO₂, enabling work with higher melt temperatures (e.g., additionalalloys, more complex casting parts). Improvements realized through theuse of fluorocarbons of the present invention as reactive cover gasescan include a significant reduction in the emission of a potentgreenhouse gas (i.e., SF₆), a potential reduction in the amount offluorine-containing reactive cover gas required to provide protection,and a reduction in total emissions. This substitution can be donewithout increasing risks for workers since the fluorocarbons of thepresent invention are all safe materials with which to work, have lowtoxicity, are nonflammable, and are not a detriment to productionequipment.

[0040] The use of perfluoroketones, or hydrofluoroketones, or mixturesthereof, in a gaseous mixture demonstrate the ability to also put outfires that are already occurring on the surface of molten magnesium.Therefore, the gases also may be used to extinguish fires on moltenmagnesium.

[0041] As discussed above, the use of a gaseous mixture comprising afluorocarbon selected from the group consisting of perfluoroketones,hydrofluoroketones, and mixtures thereof as a cover gas for handlingmolten magnesium instead of cover gases such as SF₆ provides anopportunity to reduce the emission of undesirable pollutants whileproducing similar, even increased amounts of magnesium. Accordingly, onecan use the present invention to produce protected magnesium or otherreactive metal or alloy and receive allocation of pollution credits.

[0042] In some applications, a magnesium producer can convert a facilitywhich utilizes cover gas comprising SF₆ to instead utilize a gaseousmixture comprising a fluorocarbon selected from the group consisting ofperfluoroketones, hydrofluoroketones, and mixtures thereof as a covergas. Pollution credits may be allocated according to a function of: (1)how much protected reactive metal or alloy is processed or produced; (2)how much of a reduction in emissions or use of higher GWP cover gas(e.g., SF₆) is achieved; or (3) any other recognized system. As usedherein, “allocation” of pollution credits is meant to include any systemwherein credits are awarded, assigned, designated, or otherwise creditedby any public or private agency for the processing of reactive metals oralloys.

EXAMPLES

[0043] The present invention is further illustrated, but is not meant tobe limited by, the following examples. The standard test procedure forevaluating the efficiency of each test fluorocarbon cover gas is givenbelow.

[0044] An approximately 3 kg sample of pure magnesium was placed in acylindrical steel crucible having an 11.4 cm internal diameter and washeated to 680° C. Cover gas was continuously applied to the 410 cm²surface of the molten magnesium through a 10 cm diameter ring formed of95 mm diameter stainless steel that was placed about 3 cm over themolten magnesium. The tubing was perforated on the side of the ringfacing the molten magnesium so that the cover gas flowed directly overthe molten magnesium. A square 20 cm×20 cm, 30 cm high stainless steelchamber with an internal volume of about 10.8 liters was fitted over thecrucible to contain the cover gas. The top of the chamber was fittedwith two 8.9 cm diameter quartz viewing ports and ports for a skimmingtool and thermocouple. A cover gas inlet, two gas sampling ports and adoor for adding fresh magnesium and for removing dross from the chamberwere placed on the sides of the chamber.

[0045] A stream of the cover gas was pumped from the chamber into theflow cell of an FTIR spectrophotometer (Midac I2000 Gas Phase FTIR) witha mercury cadmium telluride (MCT) detector. Using Modified ExtractiveFTIR (EPA Method 320), the volumetric concentration of HF and the testcover gas (in ppmV) were measured continuously during experimentation.Once the mixtures had stabilized, concentrations were measured over aperiod of 5 to 10 minutes, average values of these concentrations werecalculated, and those average values were used to make a relativecomparison of the test cover gases.

[0046] In all cases, initial magnesium melting was done using a standardcover gas of 0.5% SF₆ in CO₂ at a flow rate of 5.9 L/min. Theexperimental gas mixture was then substituted for the standard cover gasmixture by utilizing a train of rotameters and valves. Dry air (having a−40° C. dew point) at a flow rate of 5.9 L/min was used to create thetest cover gas by evaporating a flow of test fluid in it such that avolumetric concentration of 0.03 to 1 volume % fluorocarbon in air wasproduced.

[0047] During testing, the molten magnesium was observed for a period ofabout 20 to 30 minutes (equivalent to 10 to 15 chamber volumes exchangesof cover gas) to monitor any visible changes to the surface that wouldindicate the start of magnesium burning. The existing surface film wasthen removed by skimming the surface for about 3-5 minutes. The newsurface film that formed was then observed for a period of at 15-30minutes

[0048] The concentration of the fluorocarbon component of the cover gasmixture was started at about 1% by volume in air and reducedsequentially in steps of ½ the previous concentration to a minimumfluorocarbon concentration of 0.03 to 0.06%.

Comparative Example C1

[0049] C₄F₉OCH₃ (methoxy nonafluorobutane), a hydrofluoroether, has beendescribed as an effective fluorocarbon cover gas for molten magnesium inWorld Published Application WO 00/64614 (Example 5). In this comparativeexample, C₄F₉OCH₃ (available as NOVEC™ HFE-7100 Engineering Fluid from3M Company, St. Paul, Minn.) was evaluated as a fluorocarbon cover gasat 1% and at decreasing volumetric concentrations in air. In all cases,the volumetric flow rate for the cover gas/air mixture was 5.9 L/min. Atnominal concentrations of about 1, 0.5, 0.25 and 0.125% (corresponds to10000, 5000, 2500 and 1250 ppmV, respectively), C₄F₉OCH₃ produced a thinflexible surface film on molten magnesium immediately after skimming sothat no evidence of metal burning was observed. When the concentrationof C₄F₉OCH₃ was reduced to 0.0625% (i.e., 625 ppmV), some evidence ofburning was observed on the molten magnesium surface as white blooms,but no fire resulted. Exposure to fresh molten magnesium during skimmingcaused the HF concentration to remain essentially unchanged or to beincreased at all volumetric concentrations of C₄F₉OCH₃ tested.

[0050] The HF concentrations measured at the various volumetricconcentrations of C₄F₉OCH₃ tested are presented in TABLE 3. TABLE 3Concentration of Concentration of Concentration of C₄F_(9OCH) ₃ in AirHydrogen Fluoride over Hydrogen Fluoride over Over Molten Stable SurfaceMolten Fresh Molten Magnesium Magnesium Film Magnesium Film (ppm byvolume) (ppm by volume) (ppm by volume) 8300 4500 4100 4100 2000 22002000 980 1000 800 590 480

[0051] The data in TABLE 3 show that significant hydrogen fluoride isproduced at 800 ppm volumetric concentration of C₄F₉OCH₃ (i.e., 480-590ppm HF), the minimum concentration required to protect molten magnesiumfrom ignition.

Example 1

[0052] CF₃CF₂C(O)CF(CF₃)₂(1,1,1,2,4,4,5,5,5-nonafluoro-2-trifluoromethyl-pentan-3-one), aperfluoroketone, was evaluated as a cover gas to protect moltenmagnesium from ignition using essentially the same procedure asdescribed in Comparative Example C1 using C₄F₉OCH₃. TheCF₃CF₂C(O)CF(CF₃)₂ was prepared and purified using the followingprocedures.

[0053] Into a clean dry 600 mL Parr reactor equipped with stirrer,heater and thermocouple were added 5.6 g (0.10 mol) of anhydrouspotassium fluoride and 250 g of anhydrous diglyme (anhydrous diethyleneglycol dimethyl ether, available from Sigma Aldrich Chemical Co.). Theanhydrous potassium fluoride was spray dried, stored at 125° C. andground shortly before use. The contents of the reactor were stirredwhile 21.0 g (0.13 mol) of C₂F₅COF (approximately 95.0 percent purity)was added to the sealed reactor. The reactor and its contents were thenheated, and when a temperature of 70° C. had been reached, a mixture of147.3 g (0.98 mol) of CF₂═CFCF₃ (hexafluoropropylene) and 163.3 g (0.98mol) of C₂F₅COF was added over a 3.0 hour time period. During theaddition of the hexafluoropropylene and the C₂F₅COF mixture, thepressure was maintained at less than 95 psig (7500 torr). The pressureat the end of the hexafluoropropylene addition was 30 psig (2300 torr)and did not change over the 45-minute hold period. The reactor contentswere allowed to cool and were one-plate distilled to obtain 307.1 gcontaining 90.6% CF₃CF₂C(O)CF(CF₃)₂ and 0.37% C₆F₁₂ (hexafluoropropylenedimer) as determined by gas chromatography. The crude fluorinated ketonewas water-washed, distilled, and dried by contacting with silica gel toprovide a fractionated fluorinated ketone of 99% purity and containing0.4% hexafluoropropylene dimers.

[0054] A sample of fractionated CF₃CF₂C(O)CF(CF₃)₂ made according to theabove-described procedure was purified of hexafluoropropylene dimersusing the following procedure. Into a clean dry 600 mL Parr reactorequipped with stirrer, heater and thermocouple were added 61 g of aceticacid, 1.7 g of potassium permanganate, and 301 g of the above-describedfractionated1,1,1,2,4,4,5,5,5-nonafluoro-2-trifluoromethyl-pentan-3-one. The reactorwas sealed and heated to 60° C., while stirring, reaching a pressure of12 psig (1400 torr). After 75 minutes of stirring at 60° C., a liquidsample was taken using a dip tube, the sample was phase split and thelower phase was washed with water. The sample was analyzed usinggas-liquid chromatography (“glc”) and showed undetectable amounts ofhexafluoropropylene dimers and small amounts of hexafluoropropylenetrimers. A second sample was taken 60 minutes later and was treatedsimilarly. The glc analysis of the second sample showed no detectabledimers or trimers. The reaction was stopped after 3.5 hours, and thepurified ketone was phase split from the acetic acid and the lower phasewas washed twice with water. 261 g of CF₃CF₂C(O)CF(CF₃)₂ was collected,having a purity greater than 99.6% by glc and containing no detectablehexafluoropropylene dimers or trimers.

[0055] The perfluorinated ketone, CF₃CF₂C(O)CF(CF₃)₂, was then evaluatedas a fluorocarbon cover gas at 1% and at decreasing volumetricconcentrations in air (i.e., at about 1.0, 0.5, 0.25, 0.12, 0.06 and0.03% by volume; corresponds to 10000, 5000, 2500, 1250, 600 and 300ppm, respectively). At all concentrations tested, CF₃CF₂C(O)CF(CF₃)₂produced a thin flexible surface film on the molten magnesium duringskimming and prevented metal ignition. The film visually appeared to bethinner and more elastic than the surface film produced in the initialmolten magnesium protection using SF₆ as a cover gas and in ComparativeExample C1 using C₄F₉OCH₃ as a cover gas. The silvery-gray film producedwas stable and did not change appearance over at least 30 minutes. Thisis in contrast to the series using C₄F₉OCH₃, where evidence of metalburning was noted when the cover gas concentration was reduced to about625 ppm.

[0056] The HF concentrations measured at the various volumetricconcentrations of CF₃CF₂C(O)(CF₃)₂ tested are, presented in TABLE 4.TABLE 4 Concentration of Concentration of Concentration ofCF₃CF₂C(O)CF(CF₃)₂ Hydrogen Fluoride over Hydrogen Fluoride in Air overMolten Stable Surface Molten over Fresh Molten Magnesium Magnesium FilmMagnesium Film (ppm by volume) (ppm by volume) (ppm by volume) 10400 420670 4800 470 775 2400 360 640 1200 280 370 560 180 120 480 120 100 28040 40

[0057] The data in TABLE 2 show that, at equal volumetricconcentrations, significant less hydrogen flouride is produced usingCF₃CF₂C(O)CF(CF₃)₂ compared to C₄F₉OCH₃ as a cover gas. For example, at2000 ppm C₄F₉OCH₃, 980 ppm of HF was produced over the stable surfacefilm and 1000 ppm of HF was produced over the fresh molten film. Incontrast, at 2400 ppm CF₃CF₂C(O)CF(CF₃)₂ (a slightly higher fluorocarbonconcentration), only 360 ppm of HF was produced over the stable surfacefilm and 640 ppm of HF was produced over the fresh molten film.

[0058] In summary, the perfluorinated ketone outperformed thehydrofluoroether as a cover gas for molten magnesium (i.e. protected themolten magnesium at lower concentrations) and also generated lesshydrogen fluoride as a degradation product upon exposure to the moltenmetal surface.

[0059] Various modifications and alterations of this invention willbecome apparent to those skilled in the art without departing from thescope of this invention. Accordingly, it is to be understood that thisinvention is not to be limited to the illustrative embodiments set forthherein, but is to be controlled by the limitations set forth in thefollowing claims and any equivalents thereof.

What is claimed is:
 1. A method for generating pollution creditscomprising (a) treating molten reactive metal or alloy to protect saidmolten metal or alloy from reacting with oxygen in air by (1) providingmolten metal or alloy and (2) exposing said molten metal or alloy to agaseous mixture comprising a fluorocarbon selected from the groupconsisting of perfluoroketones, hydrofluoroketones, and mixtures thereofto yield protected metal or alloy having a protective film thereon and(b) taking allocation of pollution credits.
 2. The method of claim 1wherein said protected molten metal or alloy is selected from the groupconsisting of molten metal or alloy and solid metal or alloy.
 3. Themethod of claim 1 wherein said pollution credits are allocated accordingto a function of how much protected metal or alloy is processed.
 4. Themethod of claim 1 further comprising converting means for handlingmolten reactive metal or alloy employing SF₆ as a cover gas to means forhandling molten reactive metal or alloy employing a gaseous mixturecomprising a fluorocarbon selected from the group consisting ofperfluoroketones, hydrofluoroketones, and mixtures thereof
 5. The methodof claim 4 wherein said pollution credits are allocated according to afunction of said reduction in SF₆ usage.
 5. The method of claim 1wherein said perfluoroketone is selected from the group consisting ofCF₃CF₂C(O)CF(CF₃)₂, (CF₃)₂CFC(O)CF(CF₃)₂, CF₃(CF₂)₂C(O)CF₃)₂,CF₃(CF₂)₃C(O)CF(CF₃)₂, CF₃(CF₂)₅C(O)CF₃, CF₃CF₂C(O)CF₂CF₂CF₃,CF₃C(O)CF(CF₃₎₂, perfluorocyclohexanone, and mixtures thereof.
 6. Themethod of claim 1 wherein said fluorocarbon is a hydrofluoroketone thatis selected from the group consisting of HCF₂CF₂C(O)CF(CF₃)₂,CF₃C(O)CH₂C(O)CF₃, C₂H₅C(O)CF(CF₃)₂, CF₂CF₂C(O)CH₃, (CF₃)₂CFC(O)CH₃,CF₃CF₂C(O)CHF₂, CF₃CF₂C(O)CH₂F, CF₃CF₂C(O)CH₂CF₃, CF₃CF₂C(O)CH₂CH₃,CF₃CF₂C(O)CH₂CHF₂, CF₃CF₂C(O)CH₂CHF₂, CF₃CF₂C(O)CH₂CH₂F,CF₃CF₂C(O)CHFCH₃, CF₃CF₂C(O)CHFCHF₂, CF₃CF₂C(O)CHFCH₂F,CF₃CF₂C(O)CF₂CH₃, CF₃CF₂C(O)CF₂CHF₂, CF₃CF₂C(O)CF₂CH₂F,(CF₃)₂CFC(O)CHF₂, (CF₃)₂CFC(O)CH₂F, CF₃CF(CH₂F)C(O)CHF₂,CF₃CF(CH₂F)C(O)CH₂F, CF₃CF(CH₂F)C(O)CF₃, and mixtures thereof.
 7. Themethod of claim 1 wherein the gaseous mixture further comprises acarrier gas.
 8. The method of claim 7 wherein said carrier gas isselected from the group consisting of air, CO₂, argon, nitrogen, andmixtures thereof.